The present invention relates to an automatic transmission which comprises a power transmission mechanism incorporating a plurality of power transmission paths and a plurality of hydraulically operated frictionally engaging means. These hydraulically operated frictionally engaging means are controlled to select these power transmission paths individually, by means of hydraulic oil which is supplied through a plurality of shift control valves.
Such automatic transmissions have been known and are utilized, for example, as automatic transmissions for vehicles. Typically, an automatic transmission designed for use in a vehicle operates such that the transmission automatically controls the actuation of hydraulic clutches to change the speed change ratio in correspondence to the driving condition of the vehicle. Generally, the automatic transmission includes a speed change hydraulic unit which comprises a plurality of shift control valves, a solenoid valve to control the actuation of these shift control valves, and a manual valve operated in correspondence to the movement of the shift lever manipulated by a driver. In this arrangement, the automatic transmission automatically performs speed change control for a plurality of ranges, i.e., a reverse drive range, a neutral range and a forward drive range (D range, Second, First, etc.) which are individually selected by the operation of the manual valve operated in correspondence to the manipulation of the shift lever (generally, this automatic control is effective only in the forward drive range).
Recently, another type of automatic transmission has been introduced. This automatic transmission does not use a manual valve for the selection of a range and controls the shift of speed change ratios in each range solely by electrical signals. Such transmissions are disclosed in Japanese Laid-Open Patent Publication Nos. H5 (1993)-209683 (A) and H5 (1993)-215228 (A). Each automatic transmission disclosed there comprises a plurality of solenoid valves to control the actuation of shift control valves. The selection of the ranges, i.e., the forward drive range, the neutral range, and the reverse drive range, as well as the selection of the speed change ratios in the forward drive range are performed in correspondence to the command signals (electrical signals) which actuate these solenoid valves.
In such a speed change control system, as the selecting or switching of drive ranges and the setting of speed change ratios are controlled solely by the control pressures which are supplied from solenoid valves, if a solenoid valve malfunctions, then there can be a problem of inaccuracy in the selection of the drive ranges. As a preparation for such a problem, Japanese Laid-Open Patent Publication No. H5 (1993)-223156 (A) discloses a method for determining the drive ranges. In this method, hydraulic sensors are provided for detecting the control pressures that are generated for the respective drive ranges, and through the detection of the control pressures, the drive range selected actually at the moment is determined. In addition, drive range determination means determines the momentary drive range from the signals used for actuating the solenoid valves, and the drive range determined as presently selected from the actuation signals of the solenoid valves is compared with the drive range determined as actually selected from the detection of the control pressures, to confirm the selection of a right drive range and to determine whether there is an occurrence of abnormality.
Because a plurality of hydraulic sensors are provided to detect the command pressures that are generated for establishing the respective drive ranges, this method is likely to be affected from another problem. If any one of the hydraulic sensors malfunctions, then the determination of the drive ranges and of abnormality will become unreliable. To solve this problem, spare hydraulic sensors can be provided for each drive range. However, such solution is very costly because the sensors provided in a large number are used only for the detection of abnormality in the selection of the drive ranges.
To solve the above mentioned problem, the present invention provides a control system for an automatic transmission, which system performs a reliable determination of abnormality by determining whether a drive range is selected correctly. This system according to the present invention is capable of utilizing hydraulic pressure detecting means used for the above mentioned abnormality detection, for other purposes.
In order to achieve these objectives, the present invention provides a control system for an automatic transmission. This system comprises a power transmission mechanism (for example, the parallel shaft speed change mechanism TM of the embodiment described in the following section), a plurality of hydraulically operated frictionally engaging means (for example, the LOW clutch 11, the SECOND speed clutch 12, the THIRD speed clutch 13, the FOURTH speed clutch 14 and the FIFTH speed clutch 15 of the following embodiment), a plurality of shift control valves (for example, the first shift valve 60, the second shift valve 62, the third shift valve 64, the fourth shift valve 66, the fifth shift valve 68, the CPB valve 56 and the D inhibitor valve 58 of the following embodiment) and a forward/reverse hydraulic servomechanism (for example, the forward/reverse selection hydraulic servomechanism 70 of the following embodiment). Here, the power transmission mechanism incorporates a plurality of power transmission paths, and the hydraulically operated frictionally engaging means are activated by the supply of hydraulic pressure regulated by the shift control valves for the selection of the power transmission paths individually in the power transmission mechanism. The forward/reverse hydraulic servomechanism selects either the power transmission paths which belong to the forward drive range or the power transmission paths which belong to the reverse drive range. The control system further comprises a plurality of solenoid valves (for example, the firstxcx9cfifth on/off solenoid valves 81xcx9c85 of the following embodiment) for supplying and draining a line pressure. In this arrangement, the line pressure supplied and drained through the solenoid valves is used to actuate the shift control valves and the forward/reverse hydraulic servomechanism for selecting the power transmission paths in speed change control. The system also comprises first pressure detecting means (for example, the hydraulic switch 93 of the following embodiment) and second pressure detecting means (for example, the hydraulic switch 92 of the following embodiment). The first pressure detecting means detects the hydraulic pressure which is supplied to a forward drive servo-oil chamber (for example, the right side oil chamber 73 of the forward/reverse selection hydraulic servomechanism 70 of the following embodiment) for selecting the power transmission paths of the forward drive range. The second pressure detecting means detects the hydraulic pressure which is supplied to one of the hydraulically operated frictionally engaging means (for example, the SECOND speed clutch 12 of the following embodiment), utilizing the hydraulic pressure supplied to the forward drive servo-oil chamber, for establishing a specific speed ratio for the forward drive range. With this construction, the control system can determine whether the forward drive range is set correctly or not, based on a result of pressure detection by the first and second pressure detecting means.
In this arrangement, the first pressure detecting means detects the hydraulic pressure supplied to the forward drive servo-oil chamber of the forward/reverse hydraulic servomechanism while the second pressure detecting means detects the hydraulic pressure supplied from the forward drive servo-oil chamber to a specific hydraulically operated frictionally engaging means. Therefore, whether the line pressure is supplied to the forward drive servo-oil chamber or not is determinable from the result of the detection by either of the two pressure detecting means. Even if one of the pressure detecting means malfunctions, the monitoring of the actuation of the forward/reverse hydraulic servomechanism can be continued by the other pressure detecting means. Furthermore, because of these two pressure detecting means, the system is capable of recognizing the fault or breakdown of the pressure detecting means and the fault or breakdown of the hydraulic circuit as separate phenomena, so the system can control the transmission in a mode which is appropriate for the recognized fault or breakdown.
In addition to the detection of the actuation of the forward/reverse hydraulic servomechanism described above, the first pressure detecting means can be also used to detect the actuation of valves which control the supply of pressure to the forward drive servo-oil chamber. Also, the second pressure detecting means can be used to detect the actuation of valves which control the supply of pressure from the forward drive servo-oil chamber to a specific hydraulically operated frictionally engaging means. Therefore, the timing for setting a target speed ratio for the forward drive range can be controlled on the basis of the result of detection by the second pressure detecting means.
Preferably, a D inhibitor valve and a predetermined shift control valve (for example, the fourth shift valve 66 of the following embodiment), each of which is retainable at a respective forward drive position for the forward drive range, are arranged in this order on a servo-pressure supply line (for example, the oil passages 101b, 101e, 126 and 125 of the following embodiment) which connects a source of the line pressure (for example, the oil pump OP and the main regulator valve 50 of the following embodiment) to the forward drive servo-oil chamber. In this arrangement, the servo-pressure supply line is in fluid communication when the D inhibitor valve and the predetermined shift control valve are positioned at the respective forward drive positions, and the first pressure detecting means detects the hydraulic pressure of the servo-pressure supply line at a location between the D inhibitor valve and the predetermined shift control valve. With this arrangement, the system can monitor the actuation of the D inhibitor valve by the first pressure detecting means and can control the timing to switch from the forward drive range to a neutral range, based on the result of the detection by the first pressure detecting means.
Preferably, when the forward drive range is judged not being set correctly, based on the result of pressure detection by the first and second pressure detecting means, the control system controls the solenoid valves in accordance to modes which are predetermined for possible faults and breakdowns and thereby sets the transmission into a relief drive mode.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.